The invention relates to a picture display device comprising a picture display tube, a cathode of which is coupled to the emitter of an emitter follower drivable by a video amplifier, a negative feedback signal input of the video amplifier being coupled to an input of the emitter-follower and the collector of the emitter-follower being coupled to an input of a beam current measuring circuit, while an anti-blocking circuit is coupled to the cathode of the picture display tube to prevent blocking of the beam current measuring circuit due to leakage currents in the picture display tube.
Valvo Technische Information No. 830208 discloses a picture display device of the above-described type. The base of the emitter-follower connected to the cathode of the picture display tube is controlled in said disclosure by the negatively fed-back video amplifier. The base-emitter junction of the emitter-follower is shunted by a diode having a conductivity direction which is opposite to that of the base-emitter junction of the emitter-follower. The anti-blocking circuit is constituted by a current source which applies to the emitter of the emitter-follower a current of the order of magnitude of approximately ten micro-amperes, which ensures that the leakage current of the picture display tube cannot block this emitter follower, which would render a black level beam current measurement impossible. The beam current measuring circuit produces a control signal with which the beam current of the black levels and consequently the background colour of a picture displayed by the picture display tube is kept constant to the best possible extent. It has been found that small signal-dependent variations can still occur in these black levels and, consequently, in the background colour.